The present invention relates to a method for grinding articles, and in particular to a method for grinding the edges of ophthalmic lenses with a grinding wheel in a manner which automatically, smoothly and evenly wears the grinding surface and significantly increases the useful life of the wheel.
The present invention is particularly adapted, but not necessarily limited, to be used in connection with an apparatus for grinding the peripheries of eyeglass lenses. In one such known type of apparatus a lens is carried in a rotary work holder driven by a motor, such that the edge of the lens may engage grinding wheels driven by another motor. The work holder is mounted on a carriage for movement toward and away from the grinding wheels, as well as in directions parallel to the axis of the wheels. The edge of the rotating lens is first brought against a cylindrical outer peripheral surface of a roughing wheel to rough grind the outer periphery of the lens to a desired shape. The lens is then shifted into engagement with a V-shaped groove of a beveling wheel to form a projecting bevel on the periphery of the lens. During the beveling operation the work holder is rendered free to travel from side to side in order that the edge of the lens will be automatically centered in the groove in the wheel. The resulting bevel on the edge of the lens enables or facilitates mounting of the lens in an eyeglass frame.
While the work holder and lens are free to move axially of the beveling wheel during the beveling operation, it has been found that if axial movement is imparted to the lens during the roughing operation forces are exerted on the lens which often result in breaking, fracturing or chipping of the lens. During the roughing operation, therefore, the lens and roughing wheel are restricted against axial movement with respect to each other. In consequence, with conventional apparatus the edges of successive lenses are usually engaged with the same portion of the grinding surface of the roughing wheel, and a groove is formed in the surface.
Roughing wheels for grinding eyeglass lenses are usually comprised of an inner metal body having a cylindrical outer surface to which is bonded a layer of an abrasive material. The abrasive material, which ordinarily is diamonds in a metal matrix, has a thickness of about 0.1", and defines the cylindrical grinding surface. The width of the surface is relatively large as compared with the thickness of the lenses in order to ensure uniform contact with the entire edge surfaces of the lenses.
Although the groove forms relatively slowly in such wheels, after about 5,000 lenses have been edged the groove is ordinarily of a depth that requires retruing of the wheel. The surface of the wheel must then be reshaped or removed to the depth of the groove to again provide a cylindrical grinding surface on the wheel. Typically, such roughing wheels are capable of grinding only about 8,000 to 12,000 lenses before insufficient abrasive material remains to enable the wheel to be further retrued.
Not only are roughing wheels expensive because of the nature of the abrasive material, but retruing itself adds cost to use of the wheels. In addition, since only the center area of the wheel is ordinarily employed in the grinding operation, the side areas are never used, and in fact are removed during retruing. Consequently, despite a considerable amount of diamond abrasive material being initially provided on the wheel, when the wheel is used in a conventional apparatus only a small portion of the material is ever actually used to grind the lenses, and the remainder is wasted.
In an attempt to overcome the aforementioned disadvantages, one prior art technique contemplates providing manually operable means to axially orient the work holder and the roughing wheel relative to each other, so that the edges of successive lenses may be positioned to engage selected and different portions of the grinding surface. Ideally, an operator would orient the lenses and roughing wheel relative to each other at a frequency and in a manner to cause even wear of the grinding surface. Unfortunately, in practice such orientation is usually neglected until a groove is visible in the surface of the wheel, by which time the surface is already noncylindrical and requires reshaping. Even where the operator conscientiously changes the orientation of successive lenses and the wheel, it is unlikely that all of the various portions of the grinding surface will be engaged at a frequency and in a manner that evenly wears the wheel.
A significant advance in lens edging is disclosed in the Vulich et al U.S. Pat. No. 4,176,498, which issued Dec. 4, 1979, and is assigned to the assignee of the present invention. As taught therein, during grinding a lens is engaged with only a poriton of a grinding wheel surface, and to prevent a groove from being formed in the wheel successive lenses are automatically and periodically engaged with different portions of the wheel in a predetermined sequence of indexing across the surface. Ideally, the sequence is such that the edges of successive lenses are engaged with different portions of the grinding surface in a manner and at a frequency which ensures uniform and even wear of the entire grinding surface. Indeed, it has been found that while a grinding wheel when used in a conventional manner may be expected to grind only on the order of 10,000 lenses during its useful life, the same wheel when used to grind lenses in accordance with the teachings in the application can reasonably be expected to grind on the order of 25,000 or more lenses.
A disadvantage of the apparatus disclosed in the Vulich et al patent is that the sequence of indexing is determined on the basis of lens grinding conditions that are reasonably expected to be encountered. It occasionally happens, however, that unexpected variations are introduced into use of the apparatus which may result in uneven wear of the grinding surface. For example, in determining the number of discrete grinding positions on the surface of the wheel, it is assumed that the lenses will have some average edge thickness. Nevertheless, it is possible to encounter an extended run of either high power or low power lenses having edge thicknesses well out of tolerance with the average thickness anticipated, in which case all portions of the grinding surface will not be uniformly engaged in the predicted manner and the surface will not wear evenly. Also, while glass lenses cause wear of the wheel, for all practical purposes plastic lenses do not, so that if plastic lenses are interspersed with glass lenses it might happen that they will be edged in an order which most often engages the same portion of the grinding surface, while the glass lenses are engaged with the other portions, whereby again the wheel will not wear evenly.
It would, therefore, be extremely desirable to provide some means for automatically orienting lenses and a grinding wheel relative to each other in a manner that provides even wear of the wheel and use of all of the abrasive material thereon irrespective of the edge thickness and/or composition of the lenses, whereby to maintain a smooth grinding surface irrespective of conditions encountered without any need to rely on an operator's observations of the wear characteristics of the wheel.